brcmsmac: switch phy source files to using SPDX license identifier

With ISC license text in place under the LICENSES folder switch
to using the SPDX license identifier to refer to the ISC license.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

brcm80211: move under broadcom vendor directory

Part of reorganising wireless drivers directory and Kconfig. Note that I had to
edit Makefiles from subdirectories to use the new location.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>